1. Field of the Invention
This invention relates to solid state relays in general and, more particularly, to solid-state relays with a normally-closed contact.
2. Description of the Prior Art
The reliability, speed, quietness, and efficiency of solid-state relays make them an attractive alternative to mechanical (electromagnetically operated) relays. As a result, solid-state relays have significantly displaced mechanical relays in low and medium power applications.
Relays are typically of two basic forms, form A (normally-open) and form B (normally closed). Multiple action relays, such as single-pole, double throw, relays (form C) are a combination of the form A and form B relays.
Most solid-state relays are the normally-open, form A, (normally-off) type where the relay is non-conducting until actuated. This type of relay implemented either in monolithic form in hybrid form where the individual components are fixed to a non-conducting substrate, such as a ceramic or polyimid plate.
The normally-closed, form B, (normally-on) solid-state relay is less commonly found. Most form B relays are hybrid arrangements, as discussed above. One of the primary reasons that the normally-closed relay is not found in monolithic form is the susceptibility of the relay to transients on its output terminals when in the open state. It is known that monolithic, form B, solid-state relays become conductive if a transient of a sufficient magnitude and slew rate occurs across the output terminals of the relay in its non-conducting (actuated) state. For example, if the form B relay is controlling a motor or some other inductive load, when the relay is actuated, a transient is generated by the collapsing magnetic field within the motor. Without proper suppression of the transient (snubbing), the relay may be destroyed. Even with good snubbing, the transient may be of sufficient magnitude and slew rate to induce the relay to turn back on.
It is therefore desirable to reduce the susceptibility of normally-on solid-state relays to transients.